Context :

Aims : We probe the physical conditions in the core of Arp 299A and try to put constraints to the nature of its nuclear power source .

Methods : We used Herschel Space Observatory far-infrared and submillimeter observations of H _ { 2 } O and OH rotational lines in Arp 299A to create a multi-component model of the galaxy .
In doing this , we employed a spherically symmetric radiative transfer code .

Results : Nine H _ { 2 } O lines in absorption and eight in emission as well as four OH doublets in absorption and one in emission , are detected in Arp 299A .
No lines of the ^ { 18 } O isotopologues , which have been seen in compact obscured nuclei of other galaxies , are detected .
The absorption in the ground state OH ^ { 2 } \Pi _ { 3 / 2 } - { { } ^ { 2 } \Pi } _ { 3 / 2 } \frac { 5 } { 2 } ^ { + } - \frac { 3 } { 2 } ^ { - } doublet at 119 \mu m is found redshifted by { \sim } 175 km s ^ { -1 } compared with other OH and H _ { 2 } O lines , suggesting a low excitation inflow .
We find that at least two components are required in order to account for the excited molecular line spectrum .
The inner component has a radius of 20 - 25 pc , a very high infrared surface brightness ( \gtrsim 3 \times 10 ^ { 13 } L _ { \sun } ~ { } kpc ^ { -2 } ) , warm dust ( T _ { \mathrm { d } } > 90 K ) , and a large H _ { 2 } column density ( N _ { \mathrm { H _ { 2 } } } > 10 ^ { 24 } cm ^ { -2 } ) .
The modeling also indicates high nuclear H _ { 2 } O ( 1 - 5 \times 10 ^ { -6 } ) and OH ( 0.5 - 5 \times 10 ^ { -5 } ) abundances relative to H nuclei .
The outer component is larger ( 50 - 100 pc ) with slightly cooler dust ( 70 - 90 K ) and molecular abundances that are about one order of magnitude lower .
In addition to the two components that account for the excited OH and H _ { 2 } O lines , we require a much more extended inflowing component to account for the OH ^ { 2 } \Pi _ { 3 / 2 } - { { } ^ { 2 } \Pi } _ { 3 / 2 } \frac { 5 } { 2 } ^ { + } - \frac { 3 } { 2 } ^ { - } doublet at 119 \mu m .

Conclusions : The Compton-thick nature of the core makes it difficult to determine the nature of the buried power source , but the high surface brightness indicates that it is either an active galactic nucleus and/or a dense nuclear starburst .
Our results are consistent with a composite source .
The high OH/H _ { 2 } O ratio in the nucleus indicates that ion-neutral chemistry induced by X-rays or cosmic-rays is important .
Finally we find a lower limit to the ^ { 16 } O/ ^ { 18 } O ratio of 400 in the nuclear region , possibly indicating that the nuclear starburst is in an early evolutionary stage , or that it is fed through a molecular inflow of , at most , solar metallicity .